Television signal amplifier



ug 6, 1946 ML cAwElN TELEVISION SIGNAL AMPLIFIER 3 Sheets-Sheet l Filed March 29, 194s INVENTOR Aug. 6, 1946. T M. cAwElN A2,405,290

TELEVISION SIGNAL AMPLIFIER 1 Filed March 29, 1943 3 Sheets-Sheet 2 HG2A PLATE CURRENT '-I- GRID VOLTAGE SIGNAL VOLTAGE IO+ PLATE VOLTAGE IO CATHODE VOLTAGE TELEVISION SIGNAL AMPLIFIER Filed Marsh 29, 1943 s sheets-Sheet s INVENToR MADISON cAwErN Patented Aug.-6, 1946 UNTED STATS or rie,

2,405,290 Y 5 TELEVISION SIGNALAMPLIHER Application March 29, 1943, serial No. 480,978

This invention relates to amplifiers and particularly to amplitude discriminating amplifiers for expanding non-linearly one of the components of a composite television signal.

According' to conventional television practice, a carrier wave is modulated by both video and synchronizing components to form a composite television signal.'v At the receiver, it frequently is necessary to amplify non-linearly one of the components and sometimes to simultaneously suppress the other component. When the video component is so amplified, itis possible to eilect a gamma control such that the relative contrast between some parts of the reproduced picture diers from the relative contrasts between other parts of the picture. For example, it may be cle-- sirable to deemphasize the contrast between the darker parts of the picture while at the same time to emphasize or accentuate the contrast between the lighter parts of the picture. The 'gamma control may be accompanied by a separation of the video signal component from the synchronizing component of the composite television signal.

For the synchronous operation of the receiving apparatus, it also is necessary to separate the synchronizing and video components in such a manner that the synchronizing component is retained and the video component is suppressed. The separation of the two components of the television signal may be accomplished by means of limiter tubes. Such tubes may beV utilized to separate the two signal components and to suppress any desired one of the components Numerous circuits have been devised for operating limiter tubes. One expedient is to bias the tube for saturation at a predetermined signal level. The composite television signals are applied to the input circuit of the tube so that one of the signal components drives the tube to saturation while the other signal component is amplified in the output circuit. A disadvantage of this typeof limiter is that it can be used only in cases where the direct current signal level of the picture to be reproduced has been established.

Another type of limiter employs a tube which is biased to effect plate current cut-ofi.r The 'composite television signals are impressed upon the input circuit of the tube in such a manner that 10 Claims. (Cl. 178-7.5)

one of the signal components drives the potential of Athe grid of the tube beyond lthe point of plate current Acut-off and the other signal component is` amplified in the output lcircuit o f thetube. This type of limiter can be arranged so that the grid adjusts itself to a direct current signal level suchpthat the maximum positive peaks of one` oi the signal components produce a zero bias of the grid.

Limiters of these two types and others employed heretofore effect the desired signal separation and reproduce in amplied form, withoutk substantial distortion, the signal component which is retained,

' As .is well understood in the art, however, a disadvantage of this latter type of limiter is that it requires television signals of considerable amplitudein order that the 'signal component which is toibe suppressed have a voltage which will elect the cut-off of plate current in the tube at the desired point of the signalfwave. For example, where it is desired to recover the 'synchronizing Vcomponent to the exclusion of video componentv fromva" composite 'television signal 2 constituted in accordance with the present standards adopted by RadioV lManufacturers Association in the United States,`thegrid .voltage to eiect plate currentcut-oif should correspond to the television signals representing the black porf tions of the picture. Wherever possible it is' desirable to separate the video signal component from the synchronizing signal component in the signal Wave derived from the detector Vofithe'receiver. Hence, for the operation of a limiter of the grid cut-off xtype,'it is necessary to provide one or moreampliers for the demodulated signals in order to obtain the required amplitude in the case of weaksignals. Also, where it is desired to recover. the video component to the exclusion of the synchronizing component andto amplify non-linearly` the video component for the purpose of gamma control, it is desirable to accomplish this result before the video component is amplified.

Anobject of the present invention, therefore, is to provide,A in a'television system, an improved signal separator, the operation of which is independent of the signal strength or the directA current signal level,

Another object of the invention is to provide an ampliiier for obtaining non-linear distortion of a portion of a composite television signal, whereby the distorted portion may beV separated from other portions of the television signal.

Stilljanother object, ofthe invention is to provide, in'atelevision receiver, an improved signal limiter lwherein there is employed a yregenerative voltage feed-back 'irom the output circuit "cia triade to theginput ircuitiliereef.. whereby te television signal may be recoveredin' 'such form" to eiect a gamma control of a reproducedpicture.-

In accordance with thepresent `Vinvention, there is provided a space discharge device suclrv as a three element vacuum tube which employs a self-biasing grid circuit. Thecathode'circ'uit of the tube is made degenerative andthereis coupled to the cathode a'direct current regenerative feed-backcircuit.- The arrangement is such that the regenerative component is large relative tothe degenerative component so as to eiect a discrimination in signal amplitude, Wherebyall signals of less than a predetermined positive amplitude are suppressed inthe 'outputcircuih Alsothe signals' which are reproduced in the output circuit are amplied in a non-linear manner with respect t the corresponding signals impressed upon the input circuit.' The magnitude of the distortion varies directly as a Lfunction 4of the impressed signal7 amplitude n Whether the reproduced signals correspond to the video or synchronizing components depends upon the polarity of the composite television signal as it isimpressed upon the' input circuit of theampli'i'ler; Y

For a betterI understanding of theV invention, together withl otherand. further objects-thereof, reference" 'is had tov the" following` description taken in connection with vtheaccompanying drawings, and its scope will be pointedout inthe appended claims.

1n the accompanying `drawings Fig; 1 is a circuit diagram of a'television receiver, partially schematic, embodying .the presentlinvention, as-ernployedto recover the synchronizing component from a composite television signal;

Fig. 2 is a graphic representation of typical television signalsin relation to the platecurrent versus grid voltage characteristic of a'vacuum tube;

Fig. 3 illustrates typicalV television signal volt'- ages appearing in the circuits of Fig. 1 which are of assistance in explaining the operationl'of-l the illustrative embodiment of the invention; and

Fig. 4 is a circuit diagram of a television receiver, partially schematic, embodying the present invention as employed tof recover the video component from a composite television-*signal in a form suitable for Veffecting a gamma-control of the reproduced picture. Y N

Referring now more particularly to Fig; 1 of the drawings, there is shown schematicallya superheterodyne receiver embodying-the present invention in a preferred form. Thetelevision signals which are interceptedV by an antenna I0 are ,converted into an intermediate,` frequency and demodulated by conventional radiantvenergy receiving apparatus I I labeled inthe drawings as Frequency converter, L'. F. amplifier, detector. The rectified television signals consisting Aof the video signals andthe synchronizing impulses are ampliiiediby a video ampliferfIZ.` Theamplifled yideo signals are impressed upon the'grid 4 of a, cathode ray tube I3 for controlling, in a conventional manner, the shading of the picture to be reproduced.

The demodulated video and synchronizing signals are coupled by means of a condenser I4 to the input circuit of a space discharge device, such as a triode vacuum tube I5. The input circuit includes the cathodefand the control grid of the tube which are interconnected by resistance means. This means comprises a series arrangement of a grid leak resistor I6 and a cathode follower resistor I1, and the junction4 point of these resistors is grounded. The condenseris connected to the grid leak resist'or portion-of the input circuit resistance element. There also is provided a variable resistor I8 vconnected between the anode of the tube and the cathode follower resistor Il. A battery I9 is connected through a resistor 2B to the anode of the tube.

It Will appear from subsequent portions of the description that there is developed at the cathode of the tube I5 a, square-topped impulsive voltage at the synchronizing signal frequency. This voltage is applied to conventional synchronizing and scanning control apparatus'l, the'purpose of which is to separate the horizontal and vertical synchronizing signals so as to control the scanning generators. The horizontal` and vertical generators are connectedto*4 the Yrespective sets of deflector elements 22 and 23;'whereby'the usual defiections of the electronbeamrmay be eifected.l Y

The signal which appears in the output 'circuit of detector vI I has afor'm such as that illustrated by the curve 2E of Fig. 3; This signalv consists of video and synchronizing` components vand varies in polarity so that the negative portions thereof constitute some of the video signals'and the positive'portions thereof constitute the-remainder of the video signals and the synchronizing impulses. If such a*l signal were applied to a prior art limiter, having a' tube'biased toy ei'rect plate current cut-on, the signal would have the relationof the curve 25'iof Fig. 2, with respect to the-plate current versus gridV volt'age'characteristic of the tube I5.

This would be the case in the present ampli-A fier if it were not for the'regenerativefeed-back voltage provided. It is seenthat the tube has a zero grid-to-cathode' bias' corresponding' to 'the maximumY positive peaks vof the signal; Ifthe signal applied to the input circuit ofthe'tube is relatively weak,Y theiarnplitude is-insuiiicient to carryv the grid sufficiently' negativeV to' effect cut-01T of the' plate current below the VbasesV ci the pedestals of the synchroniaingimpulses. The result produced in such a case vis illustrated in Fig. 2 in which the curve 26 represents the plate current derived from a weak signalapplied to asignal limiter of the grid cut-off type. Thus, separation of the video and synchronizing signals by such a device is impossible, unless the signal strength is increased by amplification before the signal is applied to the limiter. Y y i The apparatus in accordance with the present invention combines the amplifying and limiting operations in the circuits associatedv with'av single three-element tube. Assume for themoment that, when a compositeftelevision signal having theY form of the'curve 24'of 3Tis1appl'ied'to the input circuity of`the'ftube I 5*,"l the'` feed-back circuit is inoperative.V The anodefvol'tage ofthe tube then would' nuctuate'asiiiustratedpyftrie curve" 21 and thecathode' voitagas shown by -the resistors Il and .29. V.using the tube of the 6J 5 type has been secured ferent by reason of the constants selected for Satisfactory operation with the resistor I1 having a value of a few thousand ohms and the resistor `having a value ofapproximately twenty-five times the value cf the resistor Ii. When using circuitelements having these constants, the .battery I9 has a potential of 300 volts.

The amplitude of the positive potential of the cathode increases as the amplitude of the signal increases in a positive direction. 'I'he amplitude of the positive anode potential decreases as the amplitude of the signal increases in a positive direction. As would Ibe expected, the voltage developed in the cathode follower resistor Il has a degenerative eiiect upon the input circuit of the-tube I5.

`Actually, however, a portion of the anode voltage is applied to the cathodeiof the tube by means of a direct current coupling comprising the vari-.- able resistor I8. This feed-back voltage has a regenerative effect upon the input circuitof the i tube and, by means of a suitable adjustment of the resistor I 8, the regenerativecornponent is made large relative to the degenerative component.

.The result produced by the regenerative feed- .back voltage is an amplitude distortion or expansion of the signal applied to the input circuit of the tube I5. Also, by reason of the described combination of regenerative and degenerative components which have materially diiierent ranges of variation, there is produced a net feed-back v voltage to the input circuit which is non-linear with respect to the signal voltage. The eifect of the feed-.back voltage is much greater in the region of the positive signals than it is in the region of the negative signals. Consequently, the- -positive portions of the signals are expanded more than the negative portions.

Also, by reason of the direct current coupling between the output and input circuits of the tube, a portion of. the positive potential of the battery I9 appears as a steady direct current voltage at the cathode. The effect of this arrangement is to shift the average bias ofthe tube further toward the point of plate current cut-oli.

f Thus, when a signal such as that represented by the curve 25 of Fig. 2 is impressed upon the resistor IS of Fig. l, the resultant expanded signal applied to the input circuit of the tube by the apparatus in accordance with this invention is represented :by the curve 29 of Fig. 2. It is seen that the synchronizing signal portions have been amplified considerably more than the video signal portions. Also, it is seen that the bases of the pedestals of the synchronizing impulses coin-` cide approximately with the point of plate current cut-oli. Such signals produce plate current in the tube I5, having the form illustrated by the curve 35. Thus, the video signals are suppressed completely in the output circuit.

The Voltage uctuations actually occurring at the anode of the tube I5 are represented by the curves 3I of Fig. 3. Similarly, the actual voltage fluctuations occurring at the cathode of the tubel are illustrated by the curves 32. Either or both of these twoV voltages may be utilized for synchronizing purposes. However, it is preferable to use the voltage derived from the cathode for the reason that it does not vary in its maximum amplitude appreciably with variations of the signal strength. The 4anode voltage has a greater .maximum amplitude variation under these, coni5 in reverse polarity to that described, the video signal component will be positive and the synchronizing signal lcomponent will be negative. By the action of the amplifierthe positive Video signals will be amplifiedA in the output circuit of the tube distorted in amplitude and the negative synchronizing signals will be suppressed. Also, since ,the magnitude of the distortion of the amplified Video signals depends upon the amplitude of these signals, the signals representing the darker .portions of the picture are distorted less than the signals representing the lighter portions of the picture. Consequently, it is seen that the ampliiier embodying the present invention is also susceptible of operation, whereby to eect a gamma control of the reproduced picture.

Such a use for the ampliiier embodying the instant invention is illustrated in Fig. 4. Circuit elements corresponding to similar `elements of Fig. 1 are given identical reference characters in the two iigures.` The demodulated composite television signals are impressed upon the input circuit of the tube I5 in such a mannerthat the video signals are of positive polarity and the synchronizing signals are of negative polarity. The output circuit including the resistor I1 is connected to the input circuit of the video amplier I2... Also theldemodulated signals, are im .device for effecting a gamma control of the reproduced picture is believed to be obvious in view of the detailed description of its operation as a limiter. By reason of the described polarity of the composite television signal as applied to the input circuit of the amplifier tube I5 of Fig. 3, the video component appears in the output circuit-in a non-linearly distorted form. Such a signal, when applied to the grid of the cathode ray tube I3, controls the reproduction of the picture in a manner to vary the contrast in the light and dark portions of the picture.

While there has been described what at present is considered the preferred vembodiment of the invention,k it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of the invention.

- What is claimed is:

1. Apparatus for effecting a non-linear ampli- Y fcation of a composite televisionsignal including video-'and synchronizing components comprising a single vacuum tube having input and output pressing said received television signal .upon the input circuit of said ampli'iier tube in such a manner that said video component is positive and said synchronizing component is negative, means for regeneratively coupling said input and outputcircuits to produce in theoutput circuit of said amplifier tube non-linearly distorted video signals, and means including said coupling to effect a gamma control of a picture reproduced from said video signals.

3. -Apparatus for separating lthe video and synchronizing components of a composite television signal comprising a vacuum` tube having an anode, a cathode and a control grid, an input circuit including said cathode and said grid, an output circuit including said Ycathode and said anode, means comprising a direct current coupling including a resistor between ,said anode and said cathode to effect a non-linear regenerative feed-back from said output circuit to said input circuit, and means including said coupling to reproduce said synchronizing component in said output circuit to the exclusion of said video component.

4. Apparatus for separating the video and synchronizing components of a composite television signal comprising, a vacuum tube having an anode, a cathode and a controlV grid, an input Y circuit including said cathode and said grid, an

output circuit including said cathode andsaid anode, means comprising a direct current coupling between said anode and "said cathode to effect a regenerative Yfeed-back-from said output circuit toY said input circuit, and means including said coupling to reproduce said synchronizing component in said output circuit to the exclusion of said video component.

5. Apparatus for separating the videoV and synchronizing components of a composite television signal comprising a vacuum tube having an anode, a cathode and a control grid, an input circuit including said cathode and said grid, an output circuit including said cathode and said anode, a resistor connected to said cathode in series with said input and output circuits, a direct current coupling including a resistor between said anode and said cathode, and means includingV said coupling to reproduce said synchronizing component in said output circuit to the exclusion of said video component.

6. Apparatus for separating the video arid synchronizing components of a Composite television signal comprising a vacuum tube having an anode, a cathode and a control grid, an input circuit including said cathode and said grid, an output circuit including said cathode and said Lanode, self-biasing means in said input circuit including a condenser in series'with said input circuit and a leak resistor in shunt with said input circuit, a resistor connected Vto said cathode in series with said input and output circuits, a direct current coupling including a resistor between said anode and said cathode, and means including saidcoupling to `reproduce said synchronizing lcomponent in said output circuit to thev exclusion of said video component.

7.' An amplifier for alternating current signals comprising a vacuum tube having an anode, a cathode and a control grida resistance element connected between said grid and said cathode, means for impressing said signals upon a portion of said resistance element, and means including a resistor for impressing upon another portion of said resistance element a potential derived from said anode, whereby to increase the `potential of said grid with respect to said cathode sufficiently to suppress in said output circuit signals of less than a predetermined positive amplitude.

8. An amplifier for alternating currentsignals comprising a vacuum tube having an anode, a cathode and a control grid, a series connection of a grid leak resistor and a cathode follower resistor, means for impressing said signals upon said grid leak resistor, and means rincluding a variable resistor for impressing upon said cathode follower resistor a potential derived from said anode to increase the potential of said grid with respect to said cathode, said anode-derived potential having a magnitude determined by said variable resistor whereby to suppress in said output circuit signals of` less than a predetermined positive amplitude.

.9.. A combined signal limiter and amplier for separating the synchronizing component from a composite television signal including video and synchronizing components comprising a single space discharge device having input and output circuits, means responsive to signals having greater than predetermined amplitudes for automatically biasing said input circuit to effect output current cut-off of negative portions of said signals, means coupling said output and input circuits responsive to signals having lesser than said predetermined amplitudes for regenerating said last named signals, and means including said coupling means, to eiiect said output current cut-oit of the video component of said lastnamed signals. Y

l0. A combined signal limiter and amplifier for separating the synchronizing component from a composite television signal including video and synchronizing components comprising a vacuum tube having input and output circuits, means including a condenser in series with said input circuit and a resistor in shunt with said input circuit and responsive to large amplitude signals for automatically biasing said input circuit to effect output current cut-off of negative portions of said signals, means including a resistor coupling said output and said input circuits for regenerating small amplitude signals, and means including said coupling to effect said output current cut-off of the video component of said small amplitude signals.

MADISON CAWEIN. 

